TROPHIC ECOLOGY OF GELATINOUS PREDATORS (CNIDARIA & CTENOPHORA) IN SAANICH INLET, VANCOUVER ISLAND, BRITISH COLUMBIA, CANADA

The present study was undertaken in order to quantify the trophic ecology of gelatinous predators (planktonic Cnidaria, Ctenophora) in Saanich Inlet, a fjord on Vancouver Island, B.C., Canada. The following topics were investigated: zooplankton seasonal abundance, biomass, and species composition; gelatinous predator chemical composition, metabolism, reproduction, feeding rates, digestion rates, gut contents analyses, and prey selection.;Trophic studies showed that gelatinous predators fed mainly on ova of E. pacifica and on copepods, although many other prey were also consumed. Considerable evidence for prey selection is presented. Feeding rates (clearance rates) were measured in the lab, as well as estimated from field data. Prey specific rates were lower in the lab but nonetheless were comparable to field values. Daily carbon rations were estimated from gut contents data and estimates of digestion rates. The results showed that small predators had an excess ration but that larger predators were often only able to obtain a maintenance ration. Predation impact, estimated using two independent methods, was low for the total mesozooplankton, but for E. pacifica ova and nauplii, reached >10% of prey stocks removed daily. Since juvenile salmon and other fish also feed extensively on these prey, it is possible that competition for food may occur.;A tentative seasonal carbon budget for gelatinous predators is presented based on metabolic rates, biomass values, in situ temperatures, assimilation estimates, and reproductive output values. The budget predicts that (TURN)25 mg C m('-3) is needed from March to August to balance gelatinous predator carbon expenditure. Of the carbon ingested, (TURN)35% is used for metabolism, (TURN)30% for production of soma, and (TURN)15% for production of gametes.;Although the trophic ecology of gelatinous predators is becoming better known, many aspects are still unstudied or incompletely quantified and require further investigations.;Zooplankton abundance and biomass were studied in the upper 25 m, using bongo nets and a plankton trap. The mesozooplankton was found to be dominated by copepods, although ova and larvae of Euphausia pacifica were seasonally important. Total mesozooplankton biomass reached 30-50 mg C m('-3) during May to July. Gelatinous predators reached 10-30 mg C m('-3) for brief periods during May and June. The predators were dominated by the ctenophore Pleurobrachia bachei and by Phialidium spp. hydromedusae. Predator production was estimated to reach (TURN)0.5 mg C m('-3) d('-1) and 5-10 mg C m('-3) y('-1). Throughout the 0-120 m water column, annual production could reach (TURN)1 g C m('-2).